Method of coating a galvanized article with iron and article produced thereby



1963 w. H MCFARLAND 3,078,555

METHOD OF COATING A GALVANIZED ARTICLE WITH IRON AND ARTICLE PRODUCED THEREBY Filed Jan. 25, 1961 IRON-ZINC ALLOY WWZLILTZ? INTERMETALLIC LAYER P Inventor WILLIAM H. M FARLAHD 15 pm,L1;9 d MQ,#M & Nag

fH-kornegs United States Patent 3,078,555 METHOD OF CQATHNG A GALVANIZED AR- TKILE WlTI-l IRON AND ARTICLE PRO- DUCED THEREBY William H. McFarland, Hobart, 1nd,, assignor to Inland Steel Company, Chicago, 111., a corporation of Dela- Ware Filed .l'an. 23, 1961, Ser. No. 84,417 19 Claims. (Cl. 2-196.5)

The present invention relates to a method of producing a zinc coating with a finish-adherent surface and more particularly to an improved method of providing a zinc coated article with a surface having good adherence for a paint-type finish.

It has been recognized that a zinc-iron alloy surface on steel sheets presently produced by the heat treatment of galvanized steel sheets is an excellent base for paint, lacquer, and other high lustre paint-type finishes. However, the intermetallic compounds forming the zinc-iron alloy, like most intermetallic compounds, are very brittle, particularly in compression, and can be pressed into many desired forms only with considerable difficulty and not without impairing the integrity of the alloy coating.

It is therefore an object of the present invention to provide a galvanized surface with good adherent properties for paint-type surface finishes.

It is a further object of the present invention to provide a galvanized sheet having good adherence for painttype surface finishes which also retain substantially all the formability characteristics of the original galvanized sheet.

it is a still further object of the present invention to provide a galvanized sheet having good adherence for paint-type surface finishes which also has good resistance to corrosion.

Other objects of the present invention will be apparent to those skilled in the art from the detailed description and claims to follow when read in conjunction with the accompanying drawing wherein:

The drawing illustrates diagrammatically an enlarged vertical sectional view of a steel sheet embodying the present invention.

It has been discovered that the foregoing express objects and other objects of the present invention apparent to those skilled in the art are achieved by providing on the outer surface of a galvanized article a thin substantially continuous layer or coating of metallic iron, preferably by applying a solution of a heat decomposable iron salt which on heating to the decomposition temperature thereof forms metallic iron without residual solid decomposition products, and heating said surface having the layer of metallic iron thereon to diffuse the metallic iron into the zinc surface of the galvanized sheet and form a surface layer comprised essentially of an iron-zinc alloy. By providing a thin surface layer of metallic iron on an outer surface of a zinc coated article and heating for a limited period, a very thin zinc-iron alloy layer is formed on the outer surface of the zinc coated article which has a thickness less than the thickness of the zinc coating and which provides very good adherence properties for painttype finishes while permitting the article to retain substantially all of its original corrosion resistance and without causing any change in the base metal as where the base is heated for a prolonged period. And, because the zinc-iron alloy layer is relatively thin, there is substantially less resistance to compression than in the conventional zinc-iron alloy surface, thereby enabling the article to be readily formed without damaging the alloy layer.

More particularly, a galvanized steel sheet product hav- "ice ing unique paint and lacquer adherence properties which also retains substantially all of its original formability and corrosion resistance properties is readily produced by providing on a surface of a galvanized steel sheet or strip a thin coating of a relatively non-volatile heat decomposable organic iron compounds, such as the oxalate, tartrate, citrate, and formate iron compounds, which when heated to the decomposition temperature thereof provides a thin surface coating of metallic iron free of other nonvolatile decomposition products on the galvanized sheet. Thereafter, the sheet is heated suificiently to alloy the metallic iron with the outer surface of the zinc. The heating is discontinued before the iron of the sheet diffuses outwardly through the galvanized surface. Since paint and lacquer adherence is substantially a surface phenomenon, it is unnecessary to have a thick zinc-iron alloy layer. Thus, only a very small amount of metallic iron need be provided on the outer surface of the zinc coating to provide the desired good finish adherence characteristics.

The iron compounds which have been found to be most useful in the present invention are the organic iron salts containing carbon and oxygen or carbon, hydrogen,

and oxygen, and particularly the oxalate, tartrate, citrate and formate salts of iron, including ferric oxalate, ferric formate, ferrous tartrate, ferrous oxalate. Other heat decomposable iron salts can be used, however, such as ferric thiocyanate and ferrocyanide. When the former iron salts are subjected to pyrolysis or thermal decomposition during the heating step of the foregoing treating process, the iron salts decompose to yield metallic iron and volatile decomposition products consisting essentially of carbon monoxide or carbon dioxide, and water vapor. The

,volatile decomposition products of the iron compounds used in the present invention, in addition to leaving the base surface without forming residual deposits, avoids the formation of objectionable oxides. Typical examples of the preferred organic iron compounds which are usefill in the present invention and the chemical equations which represent the thermal decomposition reactions which are believed to take place are as follows:

(1) Type A:

2)x' 2)y +x 2+y 2 Example:

(a) ferric oxalate pentahydrate Fe (C O.,) 5H O 2Fe+6CO -}-5H O (b) ferrous oxalate dihy-drate FeC O 2H O Fe+2CO +2l-I O (2) Type B:

M 2),.- 0 2 y- +x +y 2 Example: (a) ferrous tartrate FeC H O Fe+4C0+2H O (3) Type C:

)x( 2)y' )z +x +y 2+z 2 Example: (a) ferric formate 2Fe(CHO 2Fe+3CO+3C0 +3H O In each of the foregoing reactions, the free iron which must be first formed by the thermal decomposition of the organic salt, then alloys with the zinc metal to provide a thin outer iron-zinc alloy skin or surface on the galvanized article which is strongly adherent to surface finishes, such as paints and lacquers.

It has been found convenient to provide the required thin coating or layer of metallic iron by spraying, brushing, wiping or roll-coating a solution of a heat decomposable iron salt on a continuously moving clean galvanized sheet or strip or, if preferred, by continuously dipping the galvanized strip in a solution of the iron salt. Usually, water will be the most convenient liquid to use in preparing the solution or dispersion of iron salt. Other liquid vehicles, such as glycerine, light oils, or the like, can be used.

The quantity of the decomposable iron salt applied is not particularly critical and for a given surface finish the minimal amount which will provide the desired adherencc characteristics can be readily determined and will vary with the particular finish to be used. The concen tration of the iron salt in solution can be adjusted ac cordingly. Good paint adherence is obtained, for example, when iron is deposited on the zinc surface of a steel sheet in a concentration of about one gram per square foot of zinc coated surface. Larger amounts can, of course, be used but require longer periods of heating to effect complete alloying and generally are undesirable.

Thereafter, the article, such as a steel strip having a thickness between about 0.012 to 0.164 inch and a zinc coating from about .0004 to about .0025 inch thick and coated with a film of the iron solution uniformly deposited thereon is passed through a heated zone to decompose the iron salt and diffuse or alloy the iron with the zinc coating to form a thin outer layer of zinc-iron alloy which preferably has a thickness of about .0001 to .0002 inch. The heating step for effecting the alloy-formation can be carried out in any moderate heating apparatus, such as a luminous wall furnace or a Selas type furnace, with either an atmosphere protected or unprotected against oxidation. If there is any substantial delay be tween the deposition of the thin layer of metallic iron and the alloying thereof with the zinc coating, it is desirable to protect the iron against oxidation. In order to insure against surface oxidation, the heating step for both decomposing the iron compound and forming the zinciron alloy layer is preferably carried out in an atmosphere non-oxidizing to iron and zinc, and preferably in a neutral or reducing atmosphere.

The period of time and the temperature to which the galvanized sheet having the iron salt thereon is heated to effect decomposition of the iron salt and alloying of the resulting metallic iron with the outer surface of the zinc coating are, of course, interdependent and can vary widely. The period of time which the sheet must be held at a given temperature to effect formation of the alloy layer, however, is considerably less than the time required to form an alloy surface on a conventional galvanized sheet not having a decomposable iron solution applied thereto. The condition of the galvanized surface at the time the iron salt is applied also affects the rate at which the alloy layer is formed. Thus, if the iron solution is applied to a cold sheet of galvanized steel and the sheet placed in a heated oven having a neutral atmosphere, complete alloying of the zinc surface is effected with an oven temperature of about 1750 F. to 1780 F. within a period of about 30 seconds. If, however, the decomposable iron salt solution is applied to a continuously moving hot zinc coated strip immediately after emerging from the zinc coating pot, the iron salt is decomposed and a zinc-iron alloy is formed on the zinc coating more rapidly and within amatter of a few seconds when passed through a similarly heated oven. It is also possible, for example, to heat the galvanized sheet having the decomposable iron salt thereon for an extended period, such as 6 to 12 hours, at a temperature below the melting point of zinc, such as about 750 F. or below.

In order to further illustrate the present invention, but without limiting the invention to the particular materials and conditions employed, the following specific examples are given.

Example I A clean, oil-free, galvanized steel sheet about .0359

inch thick having a zinc coating with a thickness of about 0.001 inch was sprayed with a water solution containing 250 grams of ferric oxalate per liter. The sheet 18 held in a vertical position to drain off excess solution and provide a uniform continuous film of the heat decomposable iron salt solution on the galvanized surface thereof. The sheet was then placed in a pre-heated furnace having a temperature between about 1750 F. and 17 F. with a nitrogen atmosphere therein and mamtained therem for a period of about 30 seconds. At the end of the 30 second treating period the original bright spangled zinc surface was transformed into a characteristic light gray iron-zinc alloy surface having good paint adherence. The resultin" sheet was subjected to a standard bend test and there was no visible evidence of cracking of the alloy coating.

Example II A clean, oil-free, steel strip about .0359 inch thick and moving continually along a continuous galvanizing line adapted to deposit a zinc coating of about .0006 inch thick is spray coated on one or both sides thereof as it leaves a standard zinc coating pot with an aqueous solution of ferric tartrate containing about 300 grams of ferric tartrate per liter. After allowing the excess iron salt solution to drain from the strip leaving a thin continuous film of said solution on one side thereof, the strip is continually passed through a luminous wall heated chamber which has a reducing atmosphere and which is maintained at a temperature sufficient to heat the coated strip during its passage therethrough to just below the melting point of the zinc coating. The strip remains in the heated chamber not substantially in excess of about 5 seconds and on emerging has on the treated sides thereof a thin iron-zinc alloy layer which is diffused into the zinc coating to a depth of about .0002 inch. The galvanized strip treated in the foregoing manner has good adherence for paints and enamels and also retains its original good formability and corrosion resistance properties.

The galvanized strip treated as in Example II is found to have a further unexpected advantage over the conventional iron-zinc alloy surface in addition to the advantages previously discussed. Thus, a conventional zinc coated strip, as produced on a continuous galvanizing line, has a somewhat thicker zinc coating at the edges than at the center of the strip. When such a strip is heated in the conventional manner to form an iron-zinc alloy layer, the center portion of the strip which has a relatively thin zinc coating is transformed into the dull iron-zinc alloy form much more rapidly than the edge portions. Since the dull center portion absorbs heat more rapidly than the bright edge portions, the center portion of the strip tends to be over alloyed when the heating is continued sufficiently to alloy the edge portions of the strip. And, if the strip is heated so that the center portion is properly alloyed, the edge portions remain unalloyed. In the present invention, however, the iron salt layer is distributed substantially uniformly over the entire surface of the galvanized strip and the iron zinc alloy formation in the present invention is altogether independent of the thickness of the zinc coating. Thus, in the present invention, there is a uniform absorption of heat by the strip and no deleterious effect due to the normal variation of the thickness of the zinc coating.

While the specific examples have utilized an aqueous solution of a water soluble iron salt which is heat decomposable to provide the required metallic iron film, it should be apparent to those skilled in the art that other methods could be employed to form a thin film of metallic iron on a zinc coating. Thus, for example, a water insoluble powdered iron or iron salt which decomposes on heating to give metallic iron without leaving a solid residue can be prepared as a colloidal suspension or solution and applied as a very thin fihn so that when heated in the previously described manner forms an iron-zinc al.-

10y layer on the zinc coatedsurface.

It will also be evident that the present invention enables providing any zinc coated surface with an iron-zinc alloy surface having good adherence for a paint-type surface finish regardless of the type of metal base on which the zinc coating is applied. The ability of the present invention to provide a surface having good adherence for a paint-type finish is also not dependent on the thickness of the metal base supporting the zinc coating, on the thickness of the zinc coating, or on any variations in the thickness of the zinc coating.

In the foregoing specification and claims which follow, the term paint-type finish is intended to define generally the protective or decorative surface finishes, such as paint and enamels, which are commonly used, on metal surfaces.

Any practice of the present invention which is suggested to one skilled in the art by the foregoing disclosure is considered to be a part thereof where such practice falls within the scope of the claims appended hereto.

I claim:

1. A process of providing a galvanized article with a surface having good adherence for a paint-type finish Without substantially impairing the formability thereof which comprises, providing a substantially continuous deposit of metallic iron on a zinc coating of an article, and heating said deposit of metallic iron in contact with said zinc coating to form an exposed surface layer on said zinc coating which is comprised essentially of an ironzinc alloy substantially free of unalloyed zinc and unalloyed iron.

2. A process of providing a galvanized metal article with a surface having good adherence for a paint-type finish without substantially impairing the formability thereof which comprises, applying to the surface of a zinc coating of a metal article a substantially continuous film of a heat decomposable iron salt which forms a deposit of metallic iron without other residual solid decomposition products on heating said salt to the decomposition temperature thereof, heating said film of iron salt while in contact with said zinc coating to a temperature above the decomposition temperature of said iron salt to form a layer of metallic iron and continuing heating thereof to form a surface layer on said article comprised essentially of an iron-zinc alloy, and discontinuing said heating before any metal of the said article has diffused outwardly to said surface of said zinc coating.

3. A process as in claim 2 wherein said iron salt is an organic iron salt which when heated decomposes to form metallic iron and volatile decomposition products.

4. A process as in claim 2 wherein said iron salt is an organic iron salt selected from the group consisting of iron oxalate, iron tartrate, iron citrate and iron formate.

5. A process of providing a galvanized metal article with a surface having good adherence for a paint-type finish without substantially impairing the formability thereof which comprises, applying to the surface of a zinc coating of a metal article a uniform film of a heat decomposable organic iron salt solution which forms a deposit of metallic iron without other residual solid decomposition products on heating said salt to the decomposition temperature thereof, heating the said article in a heating zone having a temperature between about 750 F. and 1780" F., to dry said film and decompose said iron salt to form a layer of metallic iron, continuing said heating to effect alloying of said metallic iron with said zinc coating and diffuse said iron into the surface of said zinc coating, and discontinuing said heating before metal of said article has diffused outwardly to the surface of said zinc coating.

6. A process as in claim 5 wherein said solution of iron salt comprises an aqueous solution of a water soluble organic iron salt.

7. A process as in claim 6 wherein said organic iron salt is selected from the group consisting of iron oxalate, iron tartrate, iron citrate and iron formate.

8. A process as in claim 5 wherein said solution has an 6: iron' salt concentration sufiicient to deposit one gram of metallic iron per square foot of said zinc coating.

9. A process as in claim 5 wherein said zinc coating has a thickness of between about .0004 inch and about .0025 inch. 7

10. A process as in claim 5 wherein said zinc coating has a thickness of about 0.001 inch and said iron is alloyed with said zinc coated surface by diffusion therein to a depth of about 0.0001 and .0002 inch.

11. A process as in claim 5 wherein said zinc coating has a thickness of about .0006 inch and said iron is alloyed with said zinc surface by diffusion therein to a depth of about .0002 inch.

12. A process as in claim 5 wherein said metal article is a galvanized steel sheet having a thickness of between about 0.012 inch and 0.164 inch.

13. A method of applying to a galvanized metal article a strongly adherent paint finish on the surface thereof .without substantially impairing the formability thereof which comprises, providing a substantially continuous coating of metallic iron on a surface of said galvanized metal article and heating said coating of metallic iron in contact with said zinc coating to form a surface layer comprised essentially of an iron-zinc alloy, and applying a coating of a paint finish to said surface layer of ironzinc alloy.

14. A method of applying to a galvanized metal article a strongly adherent paint finish on the surface thereof without substantially impairing the formability and corrosion resistance thereof which comprises, applying on a zinc coating of a metal article a substantially continuous coating of a heat decomposable iron salt which forms a deposit of metallic iron without other solid residual decomposition products on heating to the decomposition temperature thereof, heating said coating of iron salt on said zinc coating to a temperature above the decomposition temperature of said iron salt to form a layer of metallic iron and continuing heating to diffuse said iron into said zinc coating forming a surface layer comprised essentially of an iron-zinc alloy, and discontinuing said heating before a substantial amount of metal of the said article has diffused to the surface of said zinc coating, and applying a coating of a paint finish to said surface layer of iron-zinc alloy, whereby said paint finish is strongly adherent to said galvanized metal article while said galvanized metal article retains substantially all of its original formability properties.

15. A galvanized steel sheet characterized by having good adherence for a paint-type finish while retaining the normally good formability of a galvanized steel sheet which comprises, a steel sheet having a zinc coating on a surface thereof, and an iron-zinc alloy layer forming the outer surface of said zinc coating, said iron-zinc alloy layer extending inwardly from said outer surface of said zinc coating a distance less than the thickness of said zinc coating.

16. A process of providing a galvanized article with a surface having good adherence for a paint-type finish without substantially impairing the formability thereof which comprises; providing a substantially continuous deposit of metallic iron on an exposed surface of a zinc coating of an article, heating said deposit of metallic iron in contact with said surface of the zinc coating to form thereon an exposed surface layer comprised essentially of iron-zinc alloy substantially free of unalloyed zinc and unalloyed iron, and discontinuing said heating before a substantial proportion of said zinc coating is transformed into zinc-iron alloy.

17. A process of providing a galvanized metal article with an outer surface having good adherence for a painttype finish without substantially impairing the formability thereof which comprises; applying to the surface of a zinc coating of a metal article a substantially continuous film of an iron compound which is adapted to form a surface layer of an iron-zinc alloy in contact with said zinc coating without leaving other residual solid products on heating the said compound in contact with said zinc coating, heating said iron compound in contact with said Zinc coating to form said iron-zinc alloy layer, and discontinuing said heating before any metal of said article and iron of said deposit diffuses entirely through said zinc coating.

18. A process of providing a galvanized metal article with a surface having good adherence for a paint-type finish without substantially impairing the formability thereof which comprises; applying to the surface of a zinc coating of a metal article a uniform film of a heat decomposable organic iron compound solution which forms a deposit of metallic iron without other residual compound products on heating said compound to the decomposition temperature thereof while in contact with said zinc coating, heating the said article in a heating zone having a temperature between about 750 F. and 1780 F. to effect formation of a layer of iron-zinc alloy on the surface of said zinc coating, and discontinuing said heating before metal of said article has diffused outwardly 8 through said zinc coating and before metallic iron on the outer surface of said zinc coating has diffused inwardly entirely through said zinc coating.

19. In amethod of applying to a galvanized metal article a strongly adhering paint finish on the surface thereof without substantially impairing the fcrability thereof, the improvement which comprises; providing a substantially continuous coating of metallic iron on a surface of said galvanized metal article, heating said coating of metallic iron in contact with said zinc coating to form a surface layer of iron-zinc alloy and discontinuing said heating before a substantial portion of said zinc coating is alloyed, and applying a finish coating of a paint to said layer of iron zinc alloy.

References Cited in the file of this patent FOREIGN PATENTS 5 12,963 Great Britain Sept. 29, 1939 

15. A GALVANIZED STEEL SHEET CHARACTERIZED BY HAVING GOOD ADHERENCE FOR A PAINT-TYPE FINISH WHILE RETAINING 